It drops from the women… Short before the top.
She also droped the sesalt cutlass:
https://conanexiles.gamepedia.com/Seasalt_Cutlass
This should be her: https://conanexiles.gamepedia.com/Captains_Bodyguard_Undina
I dont know if she drops the Cutlass anymore after patch. But people reported she drops the scourgestone piece.
PS.: You want to know why? Because Gall or Hekkr sometimes spawned so often… One time i got the scourgestone piece 4 time in ONE run. Gall spawned before and on top and Hekrr 2x times during going to the top.
PPS.: Did the Buccaneer's Bay Sandstone piece move?